Due to the growing concerns over problems, such as energy shortage vs. ever-increasing primary energy and electricity demand, greenhouse gas emission from conventional power plant and transportation vs. global climate change, etc., worldwide governments are investing several billion dollars to fund the development of DER generation and electrified transportation in recent years. Widespread distributed generation and charging infrastructure are under planning or construction which will bring fundamental changes to the existing distribution network structure. The power flow pattern will be changed firstly from unidirectional to bi-directional. Re-evaluation is required toward network losses, congestion levels and voltage profiles, etc. Power quality issues like harmonics and flickers will be aggravated by power electronic interfaces and fluctuating and intermittent characteristics of DERs and EV charging. The peak-valley difference will be enlarged if the generation or charging profile is not dispatchable. Existing network upgrade is indispensable to accommodate large scale DER and EV charging without proper control strategies.
In order to accommodate these changes, minimize the negative impact, and even take utilization of the controllability of distribution generation and EV charging, the primary equipment as well as the management systems of distribution network need to become smarter than before. For example, the transformers should be able to monitor the operation conditions of themselves to avoid unplanned outage; the management systems should be able to forecast the generation curve from a given wind turbine or photovoltaic (PV) panel and probability density of EV loading; and the communication system should be bi-directional for both data acquisition and supervisory control, etc. With these capabilities, we can realize the coordinated control to optimize the operation of the distribution network during both grid-connection and islanding operation.
However, for operating the distribution network with DER and EV charging infrastructure integration, there are at least three potential problems need to be solved.
1, Both DER and EV charging have fluctuating power demand characteristics. Usually the distribution transformers will not be designed according to the maximum possible power demand for cost considerations. Therefore, it is possible that the transformers will be overloaded from time to time which might accelerate the equipment aging and lead to undesired tripping. To solve this problem, both active and reactive (P/Q) power exchange among the network and DERs, EVs and the loads should be measured and controlled in a coordinated way according to the condition monitoring information of the transformers at different locations.
2, The fluctuation of DER generation and EV charging will lead to potential voltage variation and flickering. If the distribution network is connected into a strong main grid, the impacts will be relatively small. However under certain operation mode or for some rural networks, the grid connection is usually weak, the voltage fluctuation might influence the normal operation of loads or other equipment. To solve this problem, distributive EV charging infrastructure with four-quadrant operation capability can be controlled concertedly to maintain the voltage profile of the network.
3, In theory, it is feasible for a regional distribution network to be operated in the islanding mode if the local generation capacity is enough to support the local loads. Usually a synchronous generator will be switched in as the main power source to control the voltage and the frequency. The power output from DERs can help to supply partial load and reduce fuel consumption and carbon dioxide emission. However in the other hand, the intermittent output from DER will bring side effects on voltage and frequency stability of the generator. To solve this problem without sacrificing DER generation, the control of distributive EV charging infrastructure, taking into consideration of energy storage capacity from the vehicle batteries or the storage batteries, should be designed to support generator side voltage and frequency control.
Hence, a new control method or control system need to be developed to solve the above mentioned problems.